Unwrapping doesn't follow UV editor
The UV scale will depend on the scale of the selected Image Texture node in the Shader Editor, but its appearence will be stretched in the UV Editor, depending on the selected image here. If you select the same image in Shader Editor and UV Editor, the UV will appear at the right scale.
If you select 1:1 in the Shader Editor, and select 1:1 in the UV Editor, you'll see the correct scale, then if you select 3:1 in the UV Editor, it will be scaled 3 times on the X axis.
If you choose 3:1 in the Shader Editor and choose 1:1 in the UV Editor, it will look scaled 1/3 of its X scale, if you switch to 2:1 in the UV Editor, it will be 2/3 of it X scale, it will only look good if you switch to 3:1 in the UV Editor.
It works as expected, but it surprises a bit at the beginning.